Lasers are used in a wide variety of applications. In particular, lasers are integral components in optical communication systems where a beam modulated with vast amounts of information may be communicated great distances at the speed of light over optical fibers. More recently, lasers have been investigated for communicating information short distances, such as chip-to-chip in computing environments.
Of particular interest is the so-called vertical cavity surface emitting laser (VCSEL). As the name implies, this type of laser is a semiconductor micro-laser diode that emits light in a coherent beam orthogonal to the surface of a fabricated wafer. VCSELs are compact, relatively inexpensive to fabricate in mass quantities, and may offer advantages over more traditional edge emitting lasers. Edge emitting laser diodes emit coherent light parallel to the semiconductor junction layer. In contrast, VCSELs emit a coherent beam perpendicular to the boundaries between the semiconductor junction layers. Among other advantages, this may make it easier to couple the light beam to an optical fiber and may be more efficient.
FIG. 1 shows a cross-sectional view of a basic VCSEL 100 along with its symbolic representation 100′. The VCSEL 100 may include an inactive layer 101. Also shown is an active layer 102 comprising for example InGaAs, and optical confinement layers 104 comprising for example AlGaAs. These layers 102 and 104 may be sandwiched between a p-side semiconductor multi-layer reflector 106 (or p-side Distributed Bragg Reflector (DBR)), and an n-side semiconductor multi-layer reflector 108 (or n-side DBR). A resonator cavity is formed in the space between the p-DBR 106 and the n-DBR 108. A current flowing between an anode 110 and a cathode 112 excites laser oscillation such that generated laser light 114 may be emitted through a plane of the substrate 116. Of course other VCSEL configurations are possible.
As similarly discussed in, for example Reedy et al., U.S. Pat. No. 6,583,445, VCSELs may be efficiently fabricated on wafers using standard microelectronic fabrication processes and, as a result, may be integrated on-board with other components to combine both the silicon electronic and the optoelectronic devices in a single unit or integrated circuit (IC). However, since high-density CMOS electronic circuits are typically made in silicon and high performance optoelectronic devices such as VCSELs are typically made with various other optically active materials (e.g. III-V materials), such as GaAs and ZnSe, combining the two may not be entirely straight forward.
Attempts to integrate Si and III-V materials have been proposed. Heteroepitaxial growth for example involves the crystalline growth of one material on a dissimilar crystal substrate such as the heteroepitaxial growth of GaAs on silicon, and silicon on GaAs and has been done with some limited success.
More practical approaches may involve fabricating the CMOS electronic chips and optoelectronic chips separately and then later combining the two such as by epoxy casting to form what are referred to as multi-chip modules, or MCMs. Flip-chip bonding may be another approach where a chip is flipped over and attached to a substrate or other chip by a solder joint to join two dissimilar chips into intimate electrical and mechanical contact with each other to form a single module. However, when joined testing of the laser may be difficult.